Generally, the number of commutator bars (referred to as bar number), the number of rotor poles or slots (referred to a slot number) and the number of winding coils of a permanent magnet DC motor are the same. A standard commutator only sustains roughly stable commutation voltage. Even so, commutation sparks are visably generated between adjacent commutator bars as the brush pass over them. To improve commutation, sometimes the commutator bar number, the number of winding coils is twice the rotor slot number. A known micromotor adopts four poles (stator poles) and five slots, because it has the advantages of simple structure, good ventilation, less copper, etc.
A known winding arrangement for a micromotor with four poles and five slots is shown in FIG. 5. This mode has an upper winding 71 and a lower winding 72 which are successively connected with adjacent commutator bars 1-10. That is: the upper winding coil 71 and the lower winding coil 72 are wound on each tooth A-E, and each coil has two ends. For example, on tooth A, the upper winding coil 71 is connected to commutator bars 1 and 2, and the lower winding coil 72 is connected to the commutator bars 2 and 3. On tooth B, the upper winding coil 71 is connected to the commutator bars 3 and 4 while the lower winding coil 72 is connected to the commutator bars 4 and 5, and so on.
However, because the two windings are seperately connected with adjacent commutator bars in the traditional winding arrangement, when one winding is located in the best commutation position, the other winding would be ahead or behind the best position by a mechanical angle of 36 degrees, so the commutation sparks will be greater.
Another traditional winding arrangement is shown in FIG. 6. A single winding is wound in each tooth A-E, two ends of each winding are seperately connected to a pair of corresponding commutator bars 1-10. After a winding is completely wound, the next winding is reached through connecting lines known as equalizers 61-65. Five windings form a complete circuit. The current of each winding is twice the winding current of the arrangement shown in FIG. 5. Therefore, the diameter of the wire of the winding of FIG. 6 is usually twice the diameter of the wire of the winding of FIG. 5. In the winding arrangement shown in FIG. 6, the motion of the winding machine is complex and requires winding of the winding coils and winding of the equalizers. Moreover, because the equalizer lines 61-65 are required to pass full armature current, they are required to be as thick as the winding coil, and because the equalizer lines are directly connected from one commutator bar hook to another commutator bar hook they are accumulated on the neck of the commutator between the bars and the rotor core and are not beneficial to heat dissipation.
Hence there is a desire for a rotor for a DC motor which has two coils wound about each tooth of the rotor and which has improved commutation.